The present invention relates generally to multiple modules that are mounted on an alignment substrate in near perfect self-alignment with one another and with substantially no skew. More specifically, the present invention relates to an alignment substrate having a plurality of self-alignment features and a plurality of module substrates having an alignment key that connects with the self-alignment features in mating engagement so that the modules are mounted to the alignment substrate in near perfect self-alignment with one another with substantially no skew.
Thermal inkjet print cartridges operate by rapidly heating a minute volume of ink to cause the ink to vaporize and be ejected through an orifice. The ejected ink forms an ink droplet that impacts on a medium to be printed on, such as a sheet of paper. Typically, an inkjet cartridge (printhead) has a plurality of orifices arranged in an array. By properly sequencing the ejection of ink droplets from each orifice, characters or images can be printed on the medium as the ink cartridge is moved across the medium.
The minute volume of ink is contained in a vaporization chamber that includes a thin film resistor that is in contact with the ink. An electrical current from an external power supply is passed through the resistor causing the resistor to heat up. The heat from the resistor causes the ink to become superheated so that the ink explosively vaporizes and is ejected from the vaporization chamber and through the orifice to print a single ink droplet on the medium. In a thermal inkjet printer incorporating the printhead, the thin film resistors are selectively energized while a transport mechanism moves the printhead in a path across the medium to be printed on. Additionally, the medium is incrementally moved perpendicular to the path of the printhead to enable printing on almost the entire surface of the medium.
Although state-of-the-art thermal inkjet printers are fast, quite, and can produce high quality ( greater than 600 dpi) images, there are advantages to reducing or eliminating the mechanical complexity of the printer. For example, it is desirable to eliminate the noise and mechanical complexity of the printhead transport. To that end, it is known to those skilled in the printer art to reduce the mechanical complexity of a printer by mounting separate printheads in a side-by-side arrangement to form an array of discrete printheads that extend across the entire width of the medium to be printed. Selected printing elements across the array are energized to print a line of ink droplets across the medium. After each line is printed, the medium is incrementally advanced in a direction perpendicular to the array, and the line printing process is then repeated until an entire image has been printed on the medium. Consequently, the array remains stationary during printing thereby eliminating the noise, vibration, and mechanical complexity of the transport mechanism.
One disadvantage of the side-by-side arrangement is the difficulty in precisely aligning the printheads with one another. As the resolution of ink jet printers exceeds 1200 dpi (dots per inch), the alignment of the orifices of the discrete printheads across an array that can span eight or more inches requires extremely precise alignment to achieve satisfactory spacing between ejected ink droplets on the medium. Moreover, the precise alignment between the printheads must be maintained over a wide range of operating conditions, environmental conditions, and over the lifetime of the printer. Additionally, precise alignment may not be possible if one or more of the discrete printheads is manufactured with an array of orifices that are misaligned with respect to the printhead cartridge. For color printing, multiple printbars may be used for printing color images. There is one printbar for each color to be printed (typically 3 to 4 colors). Each printbar will include multiple discrete printheads with each printhead printing the same color of ink. It is critical that the printheads on the printbar be in alignment with one another, but it is also critical for the printbars to be aligned with one another. Self-alignment between the printbars is especially critical for page-wide printbars that can span 8 or more inches in width.
Prior attempts to mount multiple printheads in precise alignment with one another have included the use of a ceramic substrate as a carriers for a plurality of printheads. The printheads are fixedly mounted to the substrate using an adhesive or the like. A precision machine is required to place each printhead on the substrate, to hold each printhead in position on the substrate, and to align the substrates with one another. The required machinery is expensive and to the process for aligning the printheads on the substrate is time consuming and is subject to low process yields. Therefore, the resulting product is expensive and can not be manufactured in high volumes.
Other factors that contribute to misalignment amongst the printheads once they are mounted include a thermal mismatch between the printheads and the substrate they are mounted on. For instance environmental conditions such a temperature and humidity can cause the printheads and the substrate to thermally expand or thermally contract at different rates due to the substrate and the modules having different coefficients of thermal expansion. Furthermore, as the printheads are energized to print ink droplets, the printheads heat up. Consequently, the printheads expand at a different rate than the substrate, resulting is misalignment, thermal stress cracks, or even failure of the printhead due to thermal stress.
Accordingly, there exists a need to use standard microelectronic equipment and process steps to manufacture a substrate and multiple modules that can be mounted on the substrate with near perfect self-alignment between the modules and with substantially no skew between the modules. Moreover, there is a need for a substrate and modules that have compatible coefficients of thermal expansion so that environmental conditions do not compromise the near perfect self-alignment between the mounted modules. Finally, there is a need to mount multiple printbars in near perfect self-alignment with one another.
Broadly, the present invention is embodied in a system for precision self-alignment of multiple modules. The system includes at least one alignment substrate including at least one mounting surface and a plurality of self-alignment features having a first profile formed along the mounting surface and a plurality of module substrates, each module substrate including a base surface and an alignment key having a second profile formed along the base surface. The second profile of the alignment key complements the first profile of the self-alignment features. The module substrates are connected with the alignment substrate by inserting their respective alignment keys into the self-alignment features so that the alignment keys and the self-alignment features are connected in mating engagement with each other. As a result, the module substrates are positioned in near perfect self-alignment with one another with substantially no skew.
In one embodiment of the present invention, the first and second profiles are formed by etching the mounting and base surfaces along identical crystalline planes. The alignment substrate and the module substrates can be made from a single crystal material and a wet etch process can be used to form the first and second profiles of the self-alignment features and the alignment keys respectively.
Conventional microelectronic processing equipment and processing steps can be used to pattern and etch the self-alignment features and the alignment keys so that those features can be formed with photolithographic precision. As a result, the module substrates can be mounted to the alignment substrate with a precision that is inherent to the microelectronic process. Therefore, the need for expensive custom machinery is eliminated. Moreover, the processing steps involved in making the system according to the present invention are well understood by those skilled in the semiconductor and microelectronics art. Therefore, problems with low yield and high cost are addressed by the system of the present invention.
In another embodiment of the present invention, the alignment substrate and the module substrates are made from single crystal silicon. The use of the same material for the alignment and module substrates results in identical coefficients of thermal expansion. Accordingly, the aforementioned problems associated with thermal stress, thermal mismatch, and dissimilar coefficients of thermal expansion are addressed by the system of the present invention.
In yet another embodiment of the present invention, the alignment substrate is formed using a precision machining process to form self-alignment features with a first profile that complements the second profile of the alignment keys.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the present invention.